The present invention relates to a device for focusing a light beam. It more particularly relates to machines for welding optical fibres.
Light beam focusing device used for welding optical fibres are known from the articles by K. KINOSHITA et al, published in APPLIED OPTICS, vol 18, No. 19, October 1979, pp 3256 to 3260 and vol 21, No. 19, October 1982, pp 3419 to 3422, as well as the article by L RIVOALLAN et al published in ELECTRONICS LETTERS, vol 19, No. 2, January 1983, pp 54 and 55.
These known devices use optical elements (convergent parabolic mirrors or lenses) with a symmetry or revolution about an axis. Such elements convert a light beam with a circular cross-section into a beam of the same type and give from said light beam a focal spot, whose cross-section is also circular.
FIG. 1 shows that an optical fibre portion 2 of axis z placed in such a beam 4, whose axis x is perpendicular to axis z, absorbs a power which is a function of the surface integral, taken within the interior of a curvilinear rectangle ABCD, of the power density d in the beam, rectangle ABCD being the line of the fibre in a transverse intersection plane of the beam.
Optical fibres are generally welded by means of a laser for which the curve of the variations of the power density d, as a function of the distance of the axis x from the beam produced by said laser is of a gaussian nature.
The power received by the fibre when it moves away from the axis x of the beam along an axis y perpendicular to axes x and z and encountering the latter decreases very rapidly, due to the reduction in the area of the rectangle ABCD and the density d resulting from the moving away of the fibre.
Thus, the temperature reached by the fibre is largely dependent on the quality of the positioning of said fibre along axis y. However, in a machine used for welding the fibre and provided with a known focusing device, the fibres are generally secured by gripping or clamping their plastic protective sheath and the centering of a fibre in said sheath is not always sufficiently correct to give said fibre a good positioning tolerance along the aforementioned axis y.
In order to reduce the influence of said tolerance, it is possible to increase the diameter of the laser beam, but this leads to a considerable increase in the power of the laser for obtaining the fibre welding temperature, as well as to an elongation of the heated part of said fibres, which is prejudicial to the mechanical quality of the weld.